An in-house generation power supply system is a system which supplies power to a load by connecting the commercial electric power system and an in-house power generation device to the load in parallel. Normally, as an in-house power generation device, there is implemented an engine generator device which utilizes fossil fuel, or a solar power generator which utilizes sunlight, or the like.
However, with an in-house generation power supply system, in order to perform supply of power to the load with the power system and the in-house power generation device operating in cooperation, if the power generated by the in-house power generation device exceeds the amount of power consumed by the load, sometimes it happens that this surplus power may flow in reverse into the commercial electric power system. In this case, unless the electrical power company has made a contract to purchase this power flowing in reverse, it is necessary to ensure that this surplus power does not flow into the system.
With this objective, a converter for electrical power recovery may be provided to an in-house generation power supply system. Such a converter for electrical power recovery may comprise, for example, a device which converts the surplus power into heat and stores it thermally as hot water. A converter for electrical power recovery having this structure is a so called co-generation system. With a co-generation system, the energy which has been stored as heat may be recovered with a fuel cell system, or may be used for some other application by being conducted to a heat exchange device.
The converter for electrical power recovery described in Patent Document #1 comprises a thermal storage unit which accumulates power (comprising a heater which stores heat from the surplus power), a sensor unit which detects the power received from the system, a switching circuit which controls the input power to the thermal storage unit, and a control unit which controls the switching circuit based upon the received power detected by the sensor unit and sets the input power to the thermal storage unit. With a converter for electrical power recovery having this type of structure, since it is possible to detect with the sensor unit whether or not surplus power is being generated (by detecting the presence or absence of reverse flow in a power reception unit), when surplus power is available, it is possible to recover surplus power into the thermal storage unit by controlling the switching unit. Since the amount of power received from the system is zero when it is possible to recover the surplus power perfectly in the thermal storage unit, the output of the sensor unit also becomes zero. Accordingly, the control unit is able to decide that the surplus power is being recovered by knowing that the output of the sensor unit becomes zero.    Patent Document #1: Japanese Laid-Open Patent Publication 2004-92458.